In order to promote uniformity of equipment design, each recognized television transmission system has conformed to one of a predetermined few recognized television signal format standards. One such standard for color television is the NTSC standard. In accordance with the NTSC signal standard, for example, the spectrum reserved for each single transmission channel is six Megahertz (6 MHz). This relatively limited spectrum is required to contain the picture carrier including synchronizing information, the color subcarrier and the sound carrier. The separation between the picture carrier and the sound carrier is specified to be only 4.5 MHz, as graphed in FIG. 1.
Since video sidebands extend as high as 4.2 MHz, high frequency video energy groups lie extremely close to the sound carrier, as shown in a graph of the equivalent baseband (FIG. 2). In order to protect the aural information from video signal pollution and interference, it has been common practice to introduce a notch filter into the video path, or into the RF path at the video transmitter end of the conventional television transmission system to eliminate high frequency video energy groups at the nominal location of the sound carrier, i.e. 4.5 MHz in the NTSC system. Typical notch filter designs provide e.g. 20 DB signal reduction to the high frequency picture signal energy groups at the aural carrier center frequency.
Without this notch filter an audible "buzz" will be present and will sometimes be detected and reproduced at the receiver end. This objectionable aural "buzz" is due to interferences created by the video high frequency energy groups. Such energy groups are the result of high speed transitions in the picture content and are also the result of sharp transition edges of synchronizing pulses.
When a notch filter is in place in the video signal transmission path, the objectionable buzz is eliminated. However, the tradeoff is that the notch filter limits video bandwidth and also introduces group delay errors resulting in ringing and chroma transitional artifacts into the transmitted picture spectrum. In any instance, sound or picture will be degraded, and full advantage of the 6 MHz spectrum alloted to the NTSC channel, for example, will not be realized.
Similarly, at the conventional receiver, another notch filter centered at 4.5 MHz is provided in the received video path in order to eliminate visual interference patterns the picture due to the sound carrier. Again, picture quality will be significantly degraded by the receiver video path notch filter.
While adaptive comb filter signal processing at the transmission and reception ends of the television transmission system have dramatically improved the quality of luminance and chrominance component separation, as taught, for example, in the present inventor's U.S. Pat. No. 4,731,660, heretofore the same care has not been taken with the separation of picture and sound signal components at the transmit and receive ends of the television transmission system.
In the NTSC format, the audio signal is transmitted as frequency modulation. One characteristic of such modulation is known as the "capture effect". The capture effect means that the FM discriminator at the receiver "locks onto" the FM signal, providing it manifests a predetermined minimum signal level to achieve "quieting", and eliminates non-coherent information, such as noise and high frequency video energy groups, which is below the quieting level. Thus, for high frequency video energy groups which lie within the spectrum allotted to the sound carrier (4.5 MHz in the NTSC format) but which have amplitudes below the threshold of detection by the FM discriminator at the receiver, there is no technical reason to require removal of such picture energy groups from the transmission path. Yet, the ubiquitous notch filter at both transmitter and receiver does just that.
A hitherto unsolved need has therefore arisen for a television transmission system which makes more effective use of the available limited channel spectrum by passing lower amplitude high frequency picture signal energy groups, while minimizing interference between the picture and sound components of the television signal, and at the same time remaining fully compatible with existing transmission and reception standards and equipment.